Zoom viewfinder with front diaphragm mask



350-423 SR SEARCH ROOM July 21, 1964 w. R. LlNKE 3,141,921

ZQOM VIEWFINDER WITH FRQNT DIAPHRAGM MASK FiledDec. 14. 1959 INVENTOR.Wa ZZerR. 4 "(6 United States Patent 3,141,921 ZOOM VIEWFINDER WITHFRONT DIAPHRAGM MASK Walter R. Linke, Chicago, Ill., assignor to Bell &Howell Company, Chicago, Ill., a corporation of Illinois Filed Dec. 14,1959, Ser. No. 859,379 6 Claims. (Cl. 88--1.5)

This invention relates to a zoom viewfinder, and more particularly to azoom viewfinder in which a framing diaphragm or mask is of constantapparent size during zooming.

An object of the invention is to provide a zoom viewfinder in which aframing diaphragm is positioned in front of the front lens element adistance such that the aperture has a constant apparent size throughoutthe zooming range.

Another object of the invention is to provide a zoom viewfinder havingfixed field and eye-piece lenses and a negative lens movabletherebetween for zooming together with a front mask positioned such adistance in front of the front lens that the apparent size of the maskaperture is constant throughout the movement of the negative lens.

A complete understanding may be obtained from the following detaileddescription of a zoom view-finder forming a specific embodiment thereof,when read in conjunc tion with the appended drawing, in which the singlefigure is a longitudinal section of a zoom viewfinder constituting oneembodiment of the invention.

It is to be understood that terms front and rear as used herein referrespectively to the left and right as viewed in the drawing.

The invention provides a zoom viewfinder in which a negative componentis movable between a field lens component and an eye-piece component,and a mask or diaphragm is positioned in front of the field lenscomponent a distance such that, throughout the range of movement of thenegative component, the apparent size of the mask aperture issubstantially constant.

Referring now in detail to the drawing, there isshown therein a camera10 having an elongated, tunnel-like viewfinder chamber 11 at the top ofhousing 12, and the chamber 11 is completely enclosed along the sidesand open at the ends thereof. A zoom lens system mounted in the chamber11 comprises a fixed front or field lens positive singlet component L anaxially movable intermediate negative singlet zooming component L and afixed positive singlet eye-piece component L These components havespherical surfaces designated by radii R to R axial thicknesses t to iand axial separations s and s The component L is mounted in a lens cell13 mounted slidably on rods 14 extending parallel to the optical axis,and a flexible member 15 may be actuated by manually operable means (notshown) to move the component L to any point along the zooming range fromthe extreme telephoto position of the component L shown in full lines tothe extreme wide angle position shown in broken lines. The lens systemis substantially afocal throughout this zooming range.

To cause the apparent size of an aperture 16a in mask or diaphragm 16 tobe constant to eye 17 of a user, the mask is fixed in the position shownat a distance d from the front surface of the component L The distance dmay be derived from the following formula:

where T is the distance of the entrance pupil from the surface R for theextreme telephoto position, W is the distance of the entrance pupil fromthe surface R for the extreme wide angle position and r is the range ofmagnifica- 3,141,921 Patented July 21, 1964 tion expressed as thetelephoto magnification divided by the wide angle magnification.

Mounted at the front of the viewfinder is a weak prism 21, which isprovided for parallax compensation. A rear mask 22 is provided forsubstantially eliminating reflection. However, the aperture 16a in mask16 alone is effective to control the area of the light path, being themost restrictive of the apertures including that in the mask 22 and thechamber 11.

Preferred embodiments of the invention were constructed in compliancewith the following tables in which dimensions are in terms of inches, ndesignates the indices of refraction for the sodium D line and V theAbbe dispersion numbers:

Example I [Mask aperture 16d=.900 by .675. Zoom range=3:1. Magnificationrange=.43 to 1.30X]

3r=1.10O to .050

si=. 840 to 1. 890

Example 11 aperture 16a=.900 by .675. Zoom range=3z 1.

[Mask Magnification range:.4=3 to 1.30X

s =1.l00 to .050

8z=. 840 to 1. 890

Ri= Plano L1 na=1. 523 V=58. 6

La 7Lrl=1. 523 V=58.6

The apparent size of the mask aperture 16a remains substantiallyconstant throughout the zooming range of the above lens system, and thisis obtained with the very simple construction of the lens system.

While the invention is thus described, it is not wished to be limited tothe precise details described, as changes may be readily made withoutdeparting from the spirit of the invention.

What is claimed is:

1. In a zoom viewfinder, a fixed front component of one sign, a fixedrear component of the same sign as that of the front component, anintermediate zooming component of the sign opposite to that of the frontand rear components and movable along the optical axis to varymagnification, and a front diaphragm spaced an axial distance d from thefront surface of the front component, the viewfinder being constructedsubstantially in compliance with the following formula:

r 1 Where T is the distance of the entrance pupil from the front surfaceof the front component for the extreme telephoto position of the zoomingcomponent, W is the distance of the entrance pupil from the frontsurface of the front component for the extreme wide angle position ofthe zooming component and r is the range of magification expressed asthe quotient of the telephoto magification divided by the wide anglemagnification.

2. The zoom viewfinder of claim 1 wherein the front and rear componentsare positive and the zooming component is negative.

3. The zoom viewfinder of claim 1 wherein the front component is aconvex plano singlet convex forwardly. the rear component is a planoconvex singlet convex rearwardly, and the zooming component is abiconcave singlet predominantly concave rearwardly.

4. The zoom viewfinder of claim 1 and including a prism positioned infront of the diaphragm for parallax correction.

5. In a zoom viewfinder, proceeding from front to rear, a fixeddiaphragm mask, a fixed positive singlet front component, an axiallymovable biconcave zooming component, a fixed positive singlet rearcomponent, and being further characterized in that the viewfinderconforms substantially to the following table in which dimensions are interms of inches, and proceeding from front to rear L to L designate thelenses, R to R the radii of curvature of the surfaces, t to t the axialthicknesses, s and s the axial separations of the components, d theaxial separation of the mask and the front component, 11,, the indicesof refraction for the sodium D line and V the Abbe dispersion numbers:

d=1.040 R =+L422 L1 t1=.2l0 nd=1.523 V=58.6

R =Plano s1=l.100 to .050 R3=4.900 L; tz=.100 na=1.605 V=38.0

s2=.840 to 1.890 R =Plano L t3=.455 ns=1.523 V=58.6

6. In a zoom viewfinder, proceeding from front to rear,

a fixed diaphragm mask, a fixed positive singlet front component, anaxially movable biconcave zoom component, a fixed positive singlet rearcomponent, and being further characterized in that the viewfinderconforms substantially to the following table in which dimensions are interms of inches, and proceeding from front to rear L to L designate thelenses, R to R the radii of curvature of the surfaces, t to t;, theaxial thicknesses, s and s the axial separations of the components, dthe axial separation of the mask and the front component, n the indicesof refraction for the sodium D line and V the Abbe dispersion numbers:

d=.950 R1=+L422 L1 t1=.210 7Ld=1.523 V=58.6

Rg=Pifl11O 8t=1.100 to .050 R,=-2.349 L z2=1.100 nd=L605 V=38.0

sa=.840 to 1.890 R =Plano L3 r,=.455 n =1.523 V=58.6

References Cited in the file of this patent UNITED STATES PATENTS1,879,412 Mueller Sept. 27, 1932 2,043,900 Mihalyi June 9, 19362,859,654 Back Nov. 11, 1958 FOREIGN PATENTS 273,130 Switzerland Apr.16, 1951 294,713 Switzerland Feb. 1, 1954

1. IN A ZOOM VIEWFINDER, A FIXED FRONT COMPONENT OF ONE SIGN, A FIXEDREAR COMPONENT OF THE SAME SIGN AS THAT OF THE FRONT COMPONENT, ANINTERMEDIATE ZOOMING COMPONENT OF THE SIGN OPPOSITE TO THAT OF THE FRONTAND REAR COMPONENTS AND MOVABLE ALONG THE OPTICAL AXIS TO VARYMAGNIFICATION, AND A FRONT DIAPHRAGM SPACED AN AXIAL DISTANCE D FROM THEFRONT SURFACE OF THE FRONT COMPONENT,